1. Field of the Invention
The present invention relates to a method of fabricating a thin liquid crystal display device and, more particularly, to a method of fabricating a thin liquid crystal display device having a flat surface.
2. Discussion of the Related Art
Recently, many efforts have been made to study and develop various flat panel displays such as liquid crystal display (LCD) devices, plasma display panels (PDP), electroluminescent displays (ELD), vacuum fluorescent displays (VFD) and the like. LCDs having excellent image quality and low power consumption are most widely used.
A liquid crystal display device includes lower and upper substrates facing each other so as to leave a predetermined interval and a liquid crystal layer formed between the lower and upper substrates. Black matrix and color filter layers are formed on the upper substrate. A plurality of gate and data lines are arranged horizontally and vertically on the lower substrate, respectively, so as to define pixel areas. Pixel electrodes and thin film transistors are formed in the pixel areas.
A method of fabricating the liquid crystal display includes forming a lower substrate having thin film transistors and pixel electrodes thereon; forming an upper substrate having color filter layers; forming a panel by bonding the upper and lower substrates to each other; forming a plurality of unit panels by cutting the panel; injecting liquid crystals inside the panel; and sealing the panel.
A light-weighted and compact-sized liquid crystal display device is required for portable televisions and portable notebook computers, or the like. There are structural and technical limitations for compact sized and light-weighted devices. A glass substrate as a basic component of the liquid crystal display device is the heaviest of the components constituting the liquid display device so there is some possibility in reducing its weight. Hence, many efforts have been made to study decreasing the weight of the glass substrate.
In order to decrease the weight of the glass substrate, the glass substrate should be thinly formed. There are many opportunities to apply physical forces to the glass substrate during the process of fabricating the liquid crystal display device and the substrates undergo many heating and cooling steps. Therefore, if the glass substrate is made thinner, it is more easily broken.
Instead, after a thick glass substrate has been used at the early stage of the fabrication process, the glass substrate is thinned. Namely, devices or color filters and the like are formed on the thick glass substrate so as to prepare an upper or lower substrate. The upper and lower substrates are bonded to each other so as to form a liquid crystal display panel. An outer surface of the liquid crystal display panel is then shaved so as to reduce the thickness of the liquid crystal display device.
Wet etch using an etchant of strong acid is generally used as a method of shaving the outer surface of the liquid crystal display panel. Namely, the glass substrate is dipped in an etchant solution provided in a vessel so that the etchant solution etches the surface of the glass substrate to reduce its thickness.
A process of making a thin liquid crystal display panel according to a related art is explained as follows.
Referring to FIG. 1, an upper glass substrate 5 having color filters formed thereon and a lower glass substrate 1 having thin film transistors formed thereon are bonded to each other so as to provide a liquid crystal display panel 10. The upper and lower glass substrates 5 and 1 have the same thickness, for example about 0.7 mm.
Referring to FIG. 2, the liquid crystal device panel 10 is transferred to an etching unit so that the upper and lower glass substrates 5 and 1 are etched. Through the etch, the liquid crystal display panel 10 constituted with the glass substrates each of which is about 0.7 mm thickness becomes a liquid crystal display panel 10′ constituted with thin and light glass substrates each of which has a thickness of about 0.6 mm.
The liquid crystal display panel 10′ is transferred to a cleaning unit. In the cleaning unit, impurities attached to a surface of the liquid crystal display panel 10′ are removed using deionized water. The liquid crystal display panel is then transferred to a drying unit.
In this case, an etched thickness of the liquid crystal display panel 10 is controlled by adjusting an etching time. Namely, an etching target is set up so that the upper and lower substrates 5 and 1 of which the sum of the thickness is about 1.4 mm becomes about 1.2 mm thick, and the etch time is controlled so as to carry out the process.
In this case, there may be a chance, as shown in FIG. 3, that scratches A exist originally on the upper and lower glass substrates 5 and 1 or the scratches A are formed on the substrates 5 and 1 by an equipment during the fabrication process. If the etching step is carried out under such conditions, the substrates are etched more in the directions of depth and width of the scratches so as to bring about failure of the device.
Namely, since the extent of etching is controlled by time, the glass substrate which is etched longer enhances the depth and width of the scratches A′. Hence, if a size of the scratch becomes greater than a dot size, light passing through the liquid crystal display panel is refracted to an unwanted path so as to generate a transmittance difference. Therefore, a stain occurs on a screen of the liquid crystal display device.
In the progress of the etching step, a state variation of the substrate according to a time variance is explained by referring to FIG. 4 as follows.
FIG. 4 illustrates an etching state of a glass substrate, for example the upper glass substrate 5 in FIG. 2, in accordance with time variances (1), (2), (3) and (4) before etching is carried out and while the etching is carried out. Where ΔD0 is a depth of a scratch before etching, ΔD1 to ΔD3 indicate variations of the depth of the scratch according to an etching process, ΔW1 to ΔW3 represent variations of a width of the scratch in the etching process, and Δt is a variation of an etching thickness.
Since an energy state of the glass substrate is unstable before the etching (1), an initial reaction rate is too large when the glass substrate 5 is first contacted with the etchant. Hence, the thickness of the glass substrate 5, as shown in (2), becomes smaller so that the etching progresses abruptly in directions of the depth and width of the scratch.
Next, when the etchant continues to react with the substrate so as to make a failure portion of the scratch stable, the glass substrate 5 is etched slowly to the thickness At and the scratch portion is etched isotropically. Namely, when the scratch portion becomes stable, an etching amount of the surface of the glass substrate 5 becomes equal to that of the depth of the scratch portion so that a variation in the depth direction of the scratch becomes 0. In other words, as shown in (2), (3), and (4), ΔD1≠ΔD2≠ΔD3≠0. Yet, the etch variation in the width direction of the scratch portion increases gradually so as to have the condition of ΔW1<<ΔW2<<ΔW3.
When the glass substrate is made thin through the above-explained process, a failure size depends on and is proportional to the etching amount. Hence, a stain may occur on a screen of a liquid crystal display device.
The problems caused on the substrate by the scratches during the etching step have been explained. Besides, particles formed on the surface of the substrate may generate an etching rate difference, and the impurities generated during the etching step may form protrusions locally on the surface of the substrate. In these cases, light passing through the substrate is refracted to an unwanted path so as to degrade an image quality.
Namely, in the related art, when lower and upper glass substrates 1 and 5 having thin film transistors and color filters formed thereon, respectively are bonded to each other so as to be etched, as shown in FIG. 5, particles formed on a surface of the lower glass substrate 1 or the upper glass substrate 5 form protrusions 7 locally on the surface of the substrate. Reference numeral ‘2’ is a liquid crystal layer.
In order to overcome such problems, as shown in FIG. 6, another method is proposed in that the rest of the portion of the substrate excluding portions having the protrusions is charged with a specific material 9 so as to planarize a surface of a substrate. But, such a method fails to overcome a refractive index difference between the upper glass substrate 5 and charged material 9 so as to be unable to clear stains on a screen. Moreover, the charged material 9 increases the thickness of the substrate again.